Most experts agree that it is generally safe to take Benadryl while you are pregnant, based on the results of several studies that have not shown any clear-cut association with taking antihistamines, such as Benadryl, during the first and second trimester of pregnancy and an increased risk of birth defects or premature labour. Daily use of Benadryl throughout pregnancy or medication overdoses of Benadryl in the third trimester just before delivery may possibly increase the risk of withdrawal symptoms, such as tremors and diarrhea occurring in the newborn. If you do need to take Benadryl during pregnancy, you should only take the lowest dose for the shortest time – this means only use it occasionally, just when you need to. Also consider other natural remedies instead, such as saline nasal spray, that can also help relieve your symptoms

Benadryl is an antihistamine, and antihistamines are one of the most common categories of medicine to be taken during pregnancy. This is because they can be used to treat a wide range of conditions such as allergies, motion sickness, morning sickness, or to aid sleep.
The National Library of Medicines in their Mother to Baby Fact Sheet for diphenhydramine (Benadryl) state that the results from one study did not show an increased chance of miscarriage with the use of antihistamines, including diphenhydramine, in early pregnancy. They also say to avoid the combination of Benadryl and temazepam, because results from a single 1985 case study and animal data suggest the combination may increase the risk for stillbirth or infant death shortly after birth. They also say that it is not known if diphenhydramine exposure in a pregnancy can cause behavior or learning issues in the child. Benadryl can pass into breastmilk and may make babies irritable, sleepier or less likely to sleep. If you need an antihistamine while you are breastfeeding, it is best to take a nonsedating one like loratadine instead.